The sewer returns, depending on which floor you live on. Is there a basement?
There are several situations:
First, the horizontal pipeline has an inverted slope phenomenon, which leads to poor drainage and backwater when the displacement upstairs increases;
Second, the riser is partially blocked, especially the cast iron drainage pipe is more likely to be blocked;
Three, riser and horizontal pipe corner blockage or caused by too much sediment;
Four, there is no basement of the buried part of the discharge pipeline caused by local subsidence or pipeline collapse;
Five, the inspection well has not been unblocked for a long time, and the outdoor pipeline is not smooth;
According to the above analysis, find yourself or find a professional. You can also find property management.